1. Field of the Invention
The present invention relates to a position measuring system for determining the relative position of two objects relative to one another.
2. Discussion of Related Art
In one such incremental position measuring system, a code carrier in the form of a scale (in the case of a linear measuring system) or a graduated disk (in the case of a rotary measuring system) is associated with the first of the two objects movable relative to one another and has an elongated incremental track with a periodic line structure. The line structure of the incremental track is scanned with a scanning unit, associated with the second object, in the form of a suitable sensor system. From the output signals of the sensor system, using an evaluation unit, the amount of the displacement of the two objects relative to one another can be determined relative to the position at which the two objects were located when the position measuring system was turned on. This makes it possible (if the relative position of the objects when the position measuring system was turned on is known) to make a continuous determination of the position of the two objects relative to one another.
In many applications, it is necessary, in addition to the high-precision incremental position measurement, to make absolute position information available, to enable determining the position of the two objects relative to one another directly. To do so, it is usual to dispose an additional reference or synchronizing track on the code carrier next to the incremental track, and from this additional track the position of the objects movable relative to one another can be determined directly. However, the additional track means that increased space is required on the code carrier.
Another possible way of generating absolute position information in an incremental position measuring system is known from Japanese Patent Disclosure JP-A 59-22 6822 for an angle measuring system, which as a code carrier has a graduated disk on which an incremental track extends along its circumference. The width of the incremental track varies transversely to its longitudinal direction on the graduated disk. This variation leads to a modulation in the amplitude of the incremental signal that accordingly contains the absolute position information. Here, however, the problem is that electronically separating the incremental signal component from the absolute position component is very complicated.
An object of the present invention is to create an incremental position measuring system of the type defined at the outset in which with the simplest possible structure, absolute position information about the position of the two objects movable relative to one another can be obtained.
The above object is attained according to the invention by the creation of a position measuring system for determining the relative position of a first object and a second object movable relative to one another that includes an incremental track, associated with a first object and having a periodic line structure that has individual lines and a scanning unit, associated with a second object movable relative to the first object, that scans said periodic line structure and that generates a corresponding incremental signal. A sensor system that generates absolute position information pertaining to the relative position of the first and second objects, wherein the width of the individual lines, as measured along a longitudinal direction of the incremental track, varies over at least a portion of the breadth of the incremental track, as measured in a direction transverse to the longitudinal direction, in such a way that a structure with absolute position information is superimposed on the periodic line structure.
Accordingly, the width of the individual lines of the incremental track varies along the longitudinal direction of the incremental track in such a way that a structure with absolute position information is superimposed on the periodic line structure. The periodicity of the underlying line structure can be preserved by providing that the spacings of the longitudinal axes of the individual lines are kept constant along the longitudinal direction of the incremental track.
An embodiment of the present invention is based on the recognition that the desired absolute position information can be obtained directly from the line structure of the incremental track itself, if the width of the lines is varied while maintaining their periodic arrangement. In other words, as in a conventional incremental track, the lines are disposed at a constant spacing with respect to their individual longitudinal axes; only the width of the lines varies transversely to the longitudinal axes (hereinafter called variation of the line width). The integration of the absolute position information into the line structure of the incremental track makes an especially compact structure of the position measuring system possible, since only a single track has to be scanned.
An embodiment of the present invention appears at first glance, because of the variation in the breadth of the lines in the longitudinal direction of the incremental track, not to allow the generation of a periodic incremental signal. However, according to the present invention, there are many possible ways of varying the individual line width in such a way that with a suitable scanning system, it is readily possible both to generate the periodic incremental signal and to generate a signal containing the absolute position information.
The variation in the line width need not necessarily take place over the entire breadth of the incremental track as measured transversely to its longitudinal direction. Instead, the variation in the line width can also be provided over only part of the breadth of the incremental track. (In the claims, for the sake of conciseness and greater clarity of the claims, the breadth of the incremental track transversely to its longitudinal direction has been called the breadth of the incremental track, for short.) In a preferred embodiment of the invention, the individual lines of the incremental track each have a plurality of portions of different line width; the widths of the individual portions of a line are each selected such that all the lines of the incremental track cover essentially the same area on the code carrier. Correspondingly (given a constant spacing of the longitudinal axes of the lines), the interstices (gaps) between the adjacent lines each cover substantially the same area. Thus, there is a constant ratio of line width to gap width along the incremental track, which allows the generation of an incremental signal with a constant amplitude.
The variation of the line width is preferably formed in accordance with an oscillating function, and, in particular, a periodically oscillating function. In the case of a rotary measuring system, this readily makes it possible to generate a piece of absolute position information, if the period of the oscillating function is at least as great as the length of the incremental track. In the case of linear measuring systems, by comparison, it can be expedient to superimpose at least two and preferably three periodically oscillating functions on the incremental track, the periods of which functions differ slightly from each other, so that the absolute position information is generated by the Nonius or Vernier principle.
To create the aforementioned portions of different width of the individual lines of the incremental track, the incremental track can be subdivided transversely to its longitudinal extent into two or more partial tracks that each has a characteristic variation of the line width.
In a preferred embodiment of the present invention, the variation of the line width of the lines in the individual partial tracks in each case occurs by the same mathematical function, but there is a phase difference between different partial tracks. As mathematical functions, periodically oscillating functions, and above all the trigonometric functions, are especially suitable.
If each partial track having a certain trigonometric function has the corresponding trigonometric function phase-displaced by one-half of a period associated with it, then an offset-free sensor signal for ascertaining the absolute position can be generated by jointly evaluating the signals originating in these two partial tracks.
If for each partial track having a certain periodically oscillating function, a further partial track is provided whose trigonometric function is phase-displaced by less than one-half of a period, and in particular by one-quarter of a period, then along with the position of the two objects movable relative to one another, their direction of motion can be ascertained as well.
In an advantageous embodiment of the present invention, at least four, and preferably five partial tracks are provided, and the trigonometric functions of adjacent partial tracks are each phase-displaced relative to one another by one- quarter of a period. As a result, cross-talk between the individual channels of the sensor system that are associated with side-by-side partial tracks can be compensated for.
To generate the absolute position, each partial track of the incremental track is assigned its own sensor on the other one of the two objects movable relative to one another, and preferably two sensors spaced apart from one another in the longitudinal direction of the incremental track are provided, which are disposed on both sides of the scanning unit for generating the incremental signals.
Advantageously, both the scanning unit for generating the incremental signals and the additional sensors for generating signals with absolute position information are each formed by photoelements, to which a common light source is assigned. Since according to the present invention the incremental track can at the same time also include the absolute position information pertaining to the position of the two objects movable relative to one another, the scanning unit for generating the incremental signals and the additional sensors for generating the absolute position information can be placed close together and supplied with light from a common light source. This light source illuminates the incremental track, and from there the light (in an incident light or transmitted light process) reaches the photoreceivers.
As the scanning unit for generating the incremental signals, a sensor matrix of the kind known from International Patent Disclosure WO 97/05457 can be used in particular. International Patent Disclosure WO 97/05457 corresponds to U.S. Pat. No. 5,841,134, the entire contents of which are incorporated herein by reference. With regard to the structure of the scanning unit for generating the incremental signals, this international application is hereby incorporated by reference.
Further advantages of the invention will become clear from the ensuing description of an exemplary embodiment in conjunction with the drawings.